There's a secondary motive: I typically run my GURPS campaigns in a world that I'm still building. Typically Steampunk or Dieselpunk, it's a boundary world. The Wastelands that occupy the majority of the main continent are known for varied habitats, and extremely thin rules of reality. Magic runs rampant, and psi powers are hardly unknown. Colossal monsters, rampant automatons, and all manner of other things stalk the Wastes, and you'd be hard pressed to AVOID adventure! But there's a problem...

In GURPS, combat magic gets stomped for most purposes by technology beyond TL5. In capable hands, a standard single action revolver can get off 2 (4 if shooting with both hands on the gun and fast-cocking the hammer) shots at ~2d damage apiece, in the time it'd take a mage using default rules to prepare and throw one standard missile spell at up to 4d of damage (if he had high enough magery). This seems just barely balanced vs a Magery 2 combat mage (which is quite reasonable), until you bring rifles into the equation. A good old fashioned lever action? Suddenly the mage is trying to compete with 2 shots of 3d damage per turn. Sure, mages can bring bigger spells (explosive fireball, anyone?) to the field, at the cost of significant amounts of energy and the chance of losing control of the spell while holding it if damaged (causing it to go kaboom), but isn't too much worse than trying to use TL5 grenades.

The problem is that Steampunk is at least TL5+1, and Dieselpunk TL6+1. At this point, even a standard semi-auto pistol far outclasses the combat firepower and effectiveness of a battlemage. Rifles and automatic weapons leave the mage even further in the dust as primary combat arms. The problem, then, is how to tweak the system so as to make it feasible to play a combat mage in a TL5+ world, without being far outstripped by any random bystander with a couple points in Guns.

I've had a few thoughts, and they boil down to these:
1) allow mages to prepare spells ahead of time and store them as ammunition, similarly to the Hold Spell spell or pre-prepared Runic Magic, but slightly faster and with less penalties
2) Allow mages access to some manner of mana-focuser, that does something along the lines of doubled damage per point of energy and faster cast times on spells programmed for use with the device.
3) Just make magic ranged spells faster and/or cheaper.

I want utility magic to remain pretty much unchanged, because it has a lot of potential regardless of TL. Combat magic needs tweaking, because it simply isn't viable at higher TLs as written.

I've seen those spells, but I'm thinking in terms of what I expect players to want, and one of the things I expect a mage player to want; if they've done any sword and sorcery, they'll most likely want to be able to whip wibbly wobbly magic stuff all over the place and at least keep up with the mundanes in DPS, if not outdo them on the short term. I'm also just a bit suspicious of the balance of "I am immune to all ranged attacks, and maybe they even kill you for trying" in a setting where ranged attacks are often the norm, and are able to potentially kill in one go. I want mages to still carry guns and whatnot for backup or keeping a low (such as it is) profile, but I want offensive magic to still be worthwhile.

I'll definitely take a look at technomancer though, seems like it'll be setting appropriate.

True, but grenades aren't exactly common usually. Reading the spell, grenades thrown at the hex they're on would also work (though pitching one straight at them like a baseball would reveal the shield's existence).

If the mage is unsupported it would be a fairly effective thing to just close the distance and haul out a melee weapon of some kind (assuming you survived the discovery that they had a reverse missile shield, and the mage didn't just wreck you with spells while you tried to close). That would support my strong encouragement of my players to have at least some kind of melee skill, which I like.

Still, the ease of getting missile shield and reverse missile with even just Magery 1 makes me suspicious of them for balance purposes. It's the sort of thing that almost any mage would always have ready to go when guns are common if they even remotely think they might be shot at, so it's almost pointless to ever shoot at a mage (except as a surprise attack to prevent the Reflex spell from triggering).

Right now I'm leaning towards a technology-using spell amplifier, that only works for certain spells, and gives a damage boost or cast speed boost per point sufficient to balance it vs guns, for high TL mages who want a more direct, flashy approach to show those mundanes and their boomsticks how it's done.